1. Field of the Invention
The present invention relates to a communication system, a signal processing method, a signal processing device, and a movable body.
2. Description of the Related Art
In recent years, a GPS (Global Positioning System) receiver receiving a navigation message transmitted from an artificial satellite and calculating the current position thereof is being widely used by being applied to mobile telephones, car navigation systems, and the like.
Specifically, the navigation message transmitted from the artificial satellite contains orbit information indicating the orbit of the artificial satellite, and information such as transmission time of a signal. The GPS receiver receives the navigation messages from four or more artificial satellites, and calculates the position of each artificial satellite from the orbit information contained in the navigation message. The GPS receiver then calculates the current three-dimensional position through a simultaneous equation on the basis of the position of each artificial satellite and the difference in the transmission time and the reception time of the navigation message. In addition, the navigation messages transmitted from the four or more artificial satellites are desired when calculating the three-dimensional position. The reason is that an error exists between the clock incorporated in the GPS receiver and an atomic clock provided in the artificial satellite.
Further, each artificial satellite spreads the spectrum of data of 50 bps by using a signal termed an L1-band C/A code, that is, a pseudo-random (PRN: Pseudo-Random Noise) code, of which the code length is 1,023 and the chip rate is 1.023 MHz, and transmits the navigation message on the basis of a signal in which a carrier of 1,575.42 MHz is BPSK (Binary Phase Shift Keying) modulated by using the spread spectrum signal.
Accordingly, when the GPS receiver is intended to receive the signal transmitted from the artificial satellite, it is necessary to synchronize the PRN code, the carrier, and the data. The synchronization between the reception signal and the PRN code is achieved in a way that correlation between the PRN code of the reception signal and the PRN code which is generated in the apparatus itself is calculated, for example, by using the Fast Fourier Transform.
Japanese Patent No. 3906913 discloses a receiver capable of obtaining the entire frequency components by performing L (L is an integer) times the Fast Fourier Transform for obtaining a 1/L frequency component of the entire frequency. By using the receiver, it is possible to reduce the usage of the memory holding the result of the Fast Fourier Transform.